S690 high-strength steel welding refers to a high-performance welding process applied to quenched and tempered structural steel with yield strength ≥ 690 MPa (such as S690QL, S690QL1).

For international projects complying with the highest safety class EN 1090-2 EXC3/EXC4, S690 steel has a high carbon equivalent (CEV), which tends to form hard and brittle martensite microstructure after welding. The welding process must strictly guard against three critical risks: cold cracks (hydrogen-induced delayed cracking), softening of the heat-affected zone (HAZ), and sharp decline in low-temperature toughness caused by grain coarsening.

Complete welding technical documentation and personnel credentials must be established prior to formal production:

Conversion of pWPS to formal WPS An International Welding Engineer (IWE) compiles a Preliminary Welding Procedure Specification (pWPS), followed by Welding Procedure Qualification Record (WPQR) testing in accordance with EN ISO 15614-1. Mandatory hardness criterion: The maximum hardness of the HAZ on qualified test plates shall be strictly limited to ≤ 450 HV10. If exceeded, heat input or preheating temperature must be revised.

Welder Qualification (ISO 9606-1) All welders hold valid ISO 9606-1 certificates. The scope of qualification (welding processes including FCAW/SAW, plate thickness 8–50mm, welding positions 3G/4G, etc.) shall fully match actual production conditions.

Dual crack-resistant groove preparation (3mm mechanical machining standard) Step 1: Thermal cutting — High-precision laser or flame cutting for initial profile cutting and groove forming.
Step 2: Mechanical milling — Core crack prevention technology. A hard overheated layer forms on edges after thermal cutting. This process requires mechanical milling to remove a minimum 3mm layer from all edges, restoring a pure base metal microstructure to eliminate microcracks at the weld root.

Fit-up specification (AWS Welding Handbook 9.4) Root gap, groove angle and misalignment are controlled strictly per AWS 9.4 standards. Forced assembly is prohibited: No tensioning by external force or dead load is allowed, to minimize internal constraint stress and prevent lamellar tearing of thick steel plates during welding.

Uniform preheating before welding (100°C – 150°C, EN 1011 standard) Local heating with gas torches is strictly forbidden to avoid overheating and softening of the high-strength steel surface. Uniform electric heating shall be applied using flexible heating blankets or crawler-type ceramic heating pads. Preheating range: minimum 4 times the plate thickness on both sides of the weld (no less than 75mm), monitored at multiple points with infrared thermometers.

Low heat input and multi-pass welding control Heat input locking: Welding heat input shall be strictly controlled between 1.0 – 2.5 kJ/mm as specified in the formal WPS. Interpass temperature control: Interpass temperature is maintained at 125°C – 180°C. Welding shall be suspended and cooled down if interpass temperature exceeds 200°C. Temper bead technique: Low heat input, multi-layer and multi-pass welding is mandatory. Heat from subsequent beads provides self-tempering to prior weld layers, refining HAZ grains and preserving low-temperature impact toughness at -40°C or -60°C.

Immediate post-weld hydrogen removal (stress relief heat treatment) Hydrogen removal treatment at 200°C – 250°C: Before the weld cools to ambient temperature after welding, ceramic heating pads are reactivated to reheat the weld and HAZ to 200°C – 250°C. Slow cooling with thermal insulation blankets: Once target temperature is reached, the welded area is fully wrapped with thick asbestos insulation for natural slow cooling. This accelerates the escape of diffusible hydrogen within the weld and greatly relieves residual tensile welding stress, eliminating the microscopic driving force for delayed cracking.

Precision weld finishing standard All welds are finely ground with angle grinders to create a smooth arc transition between weld metal and base metal (transition radius ≥ 10mm). All spatter, slag, undercut and arc strikes are completely removed to avoid stress concentration caused by abrupt geometric changes.

100% Delayed Nondestructive Testing (Delayed NDT) Mandatory delay period: High-strength steel has a significant tendency for delayed cracking (hydrogen requires time to diffuse in steel). After hydrogen removal and cooling to ambient temperature, components shall stand for 24 to 48 hours before NDT: 24 hours for plate thickness ≤ 40mm; 48 hours for plates 40–50mm thick.
Inspection coverage: 100% Ultrasonic Testing (UT / Phased Array PAUT) plus 100% Magnetic Particle Testing (MT). Penetrant Testing (PT) is not permitted as a replacement for MT.

Sa2.5 shot blasting Full component shot blasting to ISO 8501-1 Sa2.5 grade for rust removal. Total elimination of pickling — Core protective control gate: To avoid catastrophic hydrogen embrittlement caused by hydrogen absorption of S690 ultra-high strength steel in pickling solution, automatic shot blasting (Sa2.5) fully replaces conventional pickling for steel structures prior to hot-dip galvanizing and paint coating.

To meet pre-commencement ITP review by clients and consultants, every critical weld must be accompanied by two quality records:

1. S690 Preheat & Post-Heat Temperature Record: Documents ceramic pad preheating temperature (100–150°C), interpass temperature, post-weld hydrogen removal temperature (200–250°C), and insulation duration.
2. S690 Welding Construction Record: Documents welder’s ISO 9606-1 certificate number, WPS reference number, actual voltage and current, calculated heat input (1.0–2.5 kJ/mm), confirmation of 3mm mechanical milling, and final linkage to 100% UT/MT reports obtained after 48 hours of delayed inspection.